Electrochemical detector cell, method and pyrolysis furnace

ABSTRACT

An electrochemical detection cell (320), which is usable either for potentiometric or electrolytic conductivity detection, has a capillary (305), which controls electrolyte flow. A gas stream containing detectable substances is input through a non-wettable plastic capillary (306). A reaction zone (310) through which both gas and liquid flow is internally wettable. Reference electrode (309) and either electrode (311) or (312) may be used for potentiometric detection. Sensor electrodes (311) and (312), both in the reaction zone, are used for conductivity detection. The electrolyte is fed through the capillary (305) gravimetrically from a reservoir, and the gas stream is supplied by a pyrolysis furnace. The mechanism of ionization in the gas phase ionization detector (GPELCD) is described.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an improved electrochemicaldetector cell, an electrochemical detection method, and a pyrolysisfurnace for use in electrochemical detection. More particularly, theinvention relates to such a cell and method which is especially adaptedfor gravimetric flow. The invention further relates to such a cell andmethod that is usable for potentiometric, coulometric or conductimetricelectrochemical detection.

2. Description of the Prior Art

Before 1950, it was common to determine the elemental composition oforganic samples by combusting them and measuring the masses of simplecompounds, such as carbon dioxide and water, produced. This method wascalled microcombustion analysis.

During the 1950s, gas chromatography (GC) was discovered, so there was alot of research in the late 1950s and early 1960s on GC instrumentation,including sensitive detectors. Thermal conductivity detectors were soonfollowed by more sensitive and selective detectors, such as hydrogenflame ionization, electron capture, argon ionization, mass spectrometricand several electrochemical detectors. The first electrochemicaldetectors for use in chromatography were described in U.S. Pat. No.3,032,493, issued May 1, 1962 to Coulson and Cavanagh. The use of theCoulson and Cavanagh detector with GC is described in Coulson et al.,"Microcoulometric Gas Chromatography of Pesticides", Agricultural andFood Chemistry, 8, 399-402 (Sept:Oct. 1960) and Coulson,"Electroanalytical Instrumentation", ISA Fall Instrument-AutomationConference, Preprint No. 181-LA-61 (Sep. 11-15, 1961).

This detection system contained essentially all of the importantfeatures of the electrolytic conductivity detectors that weresubsequently developed, including:

1) An effluent gas stream from a gas chromatograph.

2) A pyrolizer to convert organic compounds to simple substances, suchas carbon dioxide, hydrogen chloride, ammonia and water.

3) A supply of liquid supporting electrolyte.

4) A contact zone to transfer electrolytes from the gas phase to theliquid phase.

5) Electrodes in contact with the liquid phase.

6) Means for measuring the electrolyte concentration in the liquidphase.

In microcoulometry, the means for measuring is based on potentiometryand titrant generation. In electrolytic conductivity, this means issimply measurement of the ohmic resistance between two electrodes.

U.S. Pat. No. 3,158,466, issued Nov. 24, 1964 to Sternberg was the firstto report on the use of electrical conductivity in gas chromatography ofhalogenated organic compounds. The Sternberg system has all of the aboveelements for the microcoulometric detector. Sternberg did not disclose astructure for gas-liquid separation, but did suggest that electrolytesother than water may be used to achieve better sensitivity.

Almost simultaneously, Piringer, et at. reported the development of anelectrolytic conductivity detector for GC in "Construction and Operationof the Electrolytic Conductivity Detector," J. Chromatog. 8,410 (1962).They added an additional feature to the six listed above:

7) A gas-liquid separator for an electrolytic conductivity detector.

U.S. Pat. No. 3,309,845, issued Mar. 21, 1967 to Coulson, discloses anelectrolytic conductivity detector utilizing a capillary in thegas-liquid contact zone of 1 to 0.1 mm inside diameter. With the 1 mmcapillary, a liquid flow rate of approximately 1 ml. per minute wascommonly used, with lower flow rates if the contact zone had a pumpedliquid flow. This detector was provided in a commercial product byTracor. Western Scientific Associates continues to market this detectorunder license from Tracor, but with the substitution of a modified pumpflow controller.

U.S. Pat. No. 3,649,498, issued Mar. 14, 1972 to Pretorious et al.discloses a liquid and gas chromatographic detector cell in which avariety of solvents, including the lower alcohols, in addition to waterare used as a carder liquid in the detector cell.

U.S. Pat. Nos. 3,934,193 and 4,032,296, issued Jan. 20, 1976 and Jun.28, 1977 to Hall, repeat much of the above teaching and discloseunitized detector cells with structure for the physical separation ofgas and liquid phases from samples being measured by the cells.

U.S. Pat. No. 4,440,726, issued Apr. 3, 1984 to Coulson discloses areduced volume, all capillary detection cell suitable for potentiometricand coulometric detection cells. The present invention is a modificationof that detection cell.

U.S. Pat. No. 5,019,517, issued May 28, 1991 to Coulson discloses apyrolysis furnace which incorporates electrodes for detecting ioncurrents. There is a continuing need for improvement of pyrolysisfurnaces, especially for supplying gases incorporating substances toelectrochemical detection cells separate from the furnace.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a detectorcell configuration and detection method that are especially adapted toprovide a controlled flow rate of solvent liquid through the cell.

It is a further object of the invention to provide such a detector celland method in which the flow rate of the solvent liquid through the cellis controlled gravimetrically.

It is another object of the invention to provide such a detector celland method that can be used for potentiometric, coulometric orconductimetric detection.

It is still another object of the invention to provide such a detectorcell and method which is adapted to provide sensitivities in thepicogram per second range.

It is yet another object of the invention to provide such a detectorcell and method which provides a high degree of selectivity for elementsto be detected.

It is a further object of the invention to provide an improved pyrolysisfurnace especially configured for use with the detector cell.

The attainment of these and related objects may be achieved through useof the novel electrochemical detector cell, detection method andpyrolysis furnace herein disclosed. An electrochemical detector cell inaccordance with this invention has an electrolyte input flow capillarytube intersecting a second capillary tube at a given point. A gas streaminput flow capillary tube for a gas containing a substance to bedetected enters the second capillary tube proximate to the given point.The second capillary tube forms a mixing zone at the given point. Atleast one sensor electrode is positioned in the second capillary tubespaced from and above the given point. The second capillary tube has anexit for the electrolyte and the gas stream beyond the at least onesensor electrode.

An electrochemical detection method in accordance with the inventionincludes supplying an electrolyte solution through an electrolyte inputflow capillary tube to a mixing zone at a controlled flow rate. A gascontaining a detectable substance is supplied through a gas inputcapillary tube to the mixing zone. The electrolyte solution and the gasare mixed in the mixing zone. The electrolyte solution and gas arepassed from the mixing zone to a detection zone. The detectablesubstance in the electrolyte solution is detected within the detectionzone. The detector zone may be very simple and not contain a gas-liquidseparator or more complicated and have sensor electrode(s) in agas-liquid separator as has been used in previously developedelectrochemical detectors. The gas-liquid separator decreases thedetector electrical noise and also tends to cause sharp peaks tobroaden, called tailing. The electrolyte solution and gas are flowedfrom the detection zone.

A pyrolysis furnace for use with an electrochemical detection cell inaccordance with this invention has first and second concentricallypositioned electrically insulating tubes. A resistance heater is woundaround said first electrically insulating tube between said firstelectrically insulating tube and said second electrically insulatingtube.

The attainment of the foregoing and related objects, advantages andfeatures of the invention should be more readily apparent to thoseskilled in the art, after review of the following more detaileddescription of the invention, taken together with the drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section view of an electrochemical detection cell.

FIG. 2 is a cross section view of a potentiometric detectorincorporating gravimetric solvent feed.

FIG. 3 is a cross section view of a coulometric detector incorporatinggravimetric solvent feed.

FIG. 4 is a cross section view of an electrolytic conductivity detectorincorporating gravimetric solvent feed.

FIG. 5 is a cross section view of a gas-liquid separator conductimetricdetector incorporating gravimetric solvent feed.

FIG. 6 is a cross section view of a pyrolysis furnace for use with theelectrochemical detectors of FIGS. 1-5.

FIG. 7 is a cross section view similar to FIG. 6, but of anotherembodiment of a pyrolysis furnace modified for use as a gas phaseelectrolytic conductivity detector.

FIG. 8 is a cross section view of a gas phase electrolytic conductivitydetector similar to FIG. 7 in combination with a flame ionizationdetector.

FIGS. 9A and 9B and FIGS. 10A-10D are curves plotting experimentalresults obtained with the invention.

FIGS. 1-8 are in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

In connection with the present invention, the ionization mechanism ofthe detector described in my U.S. Pat. No. 5,019,517 has beenelucidated. The ionization mechanism is as follows:

If the gas phase in the detector is primarily oxidative, most of thechlorine will be completely oxidized. The chlorine will thus be in theform Cl. and Cl₂. In this description, Cl. refers to all of the oxidizedchlorine, such as ClO, ClO₂ and Cl.. The equilibrium constant forchlorine is in equation 1.

    K'=(Cl.).sup.2 /(Cl.sub.2)                                 (1)

Consequently, most of the chlorine is Cl. at low concentrations and Cl₂at high concentrations. Since only the Cl. is detected, linearsensitivity is only available at low levels. At high levels of chlorine,the sensitivity is in proportion to the square root of Cl₂, since(Cl.)=(K'*(Cl₂))⁰.5, and in between, the sensitivity gradually goes fromlinear to square root.

If reductive gases, such as hydrogen, are used in the detector, most ofthe chlorine is in the form of HCl. The amount of Cl. here is shown inEquation 2.

    K"=(Cl.)*(H.sub.2).sup.0.5 /(HCl)                          (2)

and with high concentrations of H₂, K" simply becomes K as shown inEquation 3.

    K=(Cl.)/(HCl); and (Cl.)=K(HCl)                            (3)

Thus, the concentration of Cl. is nearly linear in terms of the totalamount of chlorine in the gas phase that flows through the detector,since K is less than one tenth. At 700° to 1100° C., approximately 80 toalmost 100% of the chlorine is in the form of HCl, thus the sensitivityis lower in the reductive condition than it is in the oxidativecondition. Also, the reductive conditions yield H₂ S and NH₃, both ofwhich yield smaller currents than chlorine. The sensitivity for N and Sis on the order of one hundredth to one thousandth of the sensitivityfor Cl, Thus, the reductive detector may also be used for other elementsthan Cl, such as N, S, and phosphorus.

If high sensitivity is desired, oxidative procedures may be used. Iflinear sensitivity at all Cl. concentrations is desired, reductiveprocedures may be used. Here the sensitivity is still in the order of0.1 equivalent per mole of chlorine, while in oxidative conditions, thesensitivity may be as great as 2 or 3 equivalents per mole. Flameionization detectors (FIDs) normally have a sensitivity of 10⁻⁴equivalents per mole.

Chlorine detection is described above, and bromine is also detectedsimilarly. This detector (a gas phase electrolytic conductivitydetector, GPELCD) is selective for halogens and is insensitive tocarbon, oxygen and hydrogen.

In the detector described in U.S. Pat. No. 5,019,517, the ionization ofCl. is catalyzed on metal electrodes in the presence of alumina, orother materials, such as mullite, but not limited to such materials.Such detection can be accomplished in any detector that has the propergas phase and temperature. Thus a FID can be modified to give responseto chlorine rather than carbon. In fact, a FID can have two phases; atthe fast phase in the usual FID configuration, carbon is detected, andin the second phase, at a much higher temperature, chlorine and otherhalogens are selectively detected. In this case, chlorine is convertedprimarily to HCI using the ordinary gas composition used in the FID,which contains both hydrogen and oxygen that create H₂ O. In thepresence of water at these high temperatures, chlorine is primarily inthe form of HCl. At nanogram levels of chlorine, linear detectorresponses are observed in the gas composition that is emitted from anordinary FID.

Turning now to the drawings, more particularly to FIG. 1, there is shownan electrochemical detection cell 320, which is usable either forpotentiometric or electrolytic conductivity detection. Capillary 305,composed of Pyrex glass, controls electrolyte flow and has an insidediameter of 0.1 to 1 mm. The gas stream containing the detectablesubstances is input through a non-wettable plastic, such aspolytetrafluoroethylene, capillary 306. The reaction zone 310 throughwhich both gas and liquid flow is internally wettable, such as Pyrexglass. Reference electrode 309 and either electrode 311 or 3 12 may beused for potentiometric detection. Sensor electrodes 311 and 312, bothin the reaction zone, are used for conductivity detection. Theelectrodes should be made of platinum or other stable metals.

FIG. 2 is a potentiometric detector composed of Pyrex glass, platinumelectrodes, and other metal electrodes, for the detection ofelectrolytes such as sulfite, bromine, chloride, and other reactablechemicals in a gas stream 1 entering through capillary 2. Gravity causesthe solvent fluid 3, which is water or other solvents such as alcohols,to flow through tube 4 with an i.d. of 1 to 0.1 mm depending on the flowrate desired. Tube 4 has a length of 5 to 10 cm. The diameter ofcapillary 2, where it enters the contact zone, capillary 5, is from 1 to0.01 mm in diameter and the diameter of capillary 5 is from 2 to 0.1min. The gas stream entering capillary 5 through capillary 2 makescontact with the solvent fluid, and the gaseous compounds, which arepotential electrolytes in the gas stream, enter the solvent fluid 3 inthe contact zone. Fluids 1 and 3 both flow through the detector zone 6which has electrodes 7 and 8 inside the 2 to 0.2 mm diameter i.d.capillary 5. The flow continues up through capillary 9 into the solutionholder 10. The gas phase bubbles 11 escape out the top of holder 10. Thecell has five other electrodes. Electrode 12 is a reference electrode,such as a silver/silver halide electrode. Electrodes 13, 14, 15 and 16are two generator electrodes, a sensor electrode, and a referenceelectrode, respectively, for coulometric maintenance of constantconcentration of a reactive chemical, such as bromine, in the bulkelectrolyte solution in holder 10.

When sulfur dioxide, or any other reducing agent, enters the contactzone 5 in the gas stream, the oxidant (bromine) is decreased and thepotential difference between electrodes 12 and 7 or 8 changes. Thispotential difference is recorded to detect and measure the amount ofreactant in the gas stream. Other oxidants, such as iodine, may be usedin place of bromine. Reactions such as complexation or precipitation maybe used instead of redox reactions in this system. For example, silverions may be used to precipitate chloride or other halides in the contactzone.

FIG. 3 is a coulometric detector composed of Pyrex glass, platinumelectrodes, and other metal electrodes, for the detection ofelectrolytes such as sulfite, bromine, chloride, and other reactablechemicals in a gas stream 101 entering through capillary 102. Gravitycauses the solvent fluid 103, which is water and electrolytes or othersolvents such as acetic acid, to flow through tube 104 with an i.d. of 1to 0.1 mm depending on the flow rate desired. Tube 104 has a length of 5to 10 cm. The diameter of capillary 102, where it enters the contactzone, capillary 105, is from 1 to 0.01 mm in diameter and the diameterof capillary 105 is from 2 to 0.1 min. The gas stream entering capillary105 through capillary 102 makes contact with the solvent fluid and thegaseous compounds,, which are potential electrolytes in the gas stream,enter the solvent fluid 103 in the contact zone. Fluids 101 and 103 bothflow upward through the detector zone 106 which has electrodes 107 and108 inside the 2 to 0.2 mm diameter i.d. capillary 105. The flowcontinues up through capillary 109 into the solution holder 110. The gasphase bubbles 111 escape out the top of holder 110. Electrode 112 is areference electrode, such as a silver/silver halide electrode. Theelectrolytic solution flows upward through capillaries 115 and 116.Electrodes 113 and 114 are two generator electrodes. The rate ofgeneration is controlled by the sensor electrode 107 and/or 108 andreference electrode 112 for coulometric maintenance of a constantconcentration of a reactive chemical, such as bromine, in theelectrolyte solution in capillary 105.

When sulfur dioxide, or other reducing agent, enters the contact zone106 in the gas stream, the oxidant (bromine) is decreased and thepotential difference between electrodes 112 and 107 and/or 108 changes.This potential difference results in an increase in the generation rateof the titrant, which is recorded to detect and measure the amount ofreactant in the gas stream. Other oxidants, such as iodine, may be usedin place of bromine. Reactions such as complexation or precipitation maybe used instead of redox reactions in this system. For example, silverions may be used to precipitate chloride or other halides in the contactzone.

FIG. 4 is a conductimetric detector composed of Pyrex glass, platinumelectrodes, and other metal electrodes for the detection of electrolytessuch as hydrogen chloride, sulfur dioxide and other chemicals in a gasstream 201 entering through capillary 202. Gravity causes the solventfluid 203, which is water or other solvents such as alcohols, to flowthrough the deionizer 212 and tube 204 with an i.d. of 1 to 0.1 mmdepending on the flow rate desired. Tube 204 has a length of 5 to 10 cm.The diameter of capillary 202, where it enters the contact zone,capillary 205, is from 1 to 0.01 mm in diameter and the diameter ofcapillary 205 is from 2 to 0.1 min. The gas stream entering capillary205 through capillary 202 makes contact with the solvent fluid and thegaseous compounds, which are potential electrolytes in the gas stream,enter the solvent fluid 203 in the contact zone. Fluids 201 and 203 bothflow through the detector zone 206, which has electrodes 207 and 208inside the 2 to 0.2 mm diameter i.d. capillary 205. The flow continuesup through capillary 209 into the solution holder 210. The gas phasebubbles 211 escape out the holder 210. Electrolytic conductivity betweenelectrodes 207 and 208 is measured and recorded as a function of time.

FIG. 5 is an electrical conductimetric detector similar to the detectorof FIG. 4, but with a gas-liquid separator. Gas stream 501 entersthrough capillary 502 and contacts the liquid 503 flowinggravimetrically through deionizer 512 and flow controller capillary 504to contact zone 505. The screens 513 and 514 hold the deionizerparticles in place. Capillary 506, the detector zone with electrodes 507and 508 near the gas-liquid separation point at the top of capillary506, has liquid flow only. A small portion of the liquid and all of thegas pass capillary 506 to capillary 515. All of the liquid, as droplets509, and all of the gas flow up capillary 515, entering the liquidreservoir 510. The gas bubbles 511 escape out the top and the liquidrecycles. In its preferred form, the detector of FIG. 5 has a height ofabout 6.5 inches.

The gas stream entering any of the detectors shown in FIGS. 1-5 may betreated in a pyrolysis furnace as shown in FIG. 6 to convert organiccompounds to potential electrolytes for detection. A special miniaturefurnace, that has a temperature controlled to within one degreecentigrade at temperatures up to 1100 deg. centigrade, composed of firstand second fused quartz electrically insulating tubes 400 and 402, aplatinum heater coil 404, with a platinum-rhodium thermocouple 406inside the pyrolysis furnace has very little field effect in pyrolysiszone 408 if the first end 410 of the heating coil 404 is wound fromfight to left and back again to the second end 412. The heating coil 404is wound for a length of approximately twelve centimeters on the outsideof a 9 mm o.d. by 7 mm i.d. fused quartz electrically insulating tube402, that has the thermocouple 406 inside and a 13 mm o.d. by 11 mm i.d.fused quartz electrically insulating tube 400 outside the heater coil.Fused quartz powder 414 is used to keep the platinum heater coil 404from becoming shorted by filling the space between the two quartz tubes400 and 402 with the powder 414. Alumina tubes may be used instead offused quartz if higher temperatures are needed. Thermal insulation 416is placed outside of the 13 mm o.d. tube 400. For less accuratetemperature control, the thermocouple may be placed in a tube outside ofthe heating coil, such as tube 418. Presently available furnaces usuallyuse an outside thermocouple location.

The furnace in FIG. 6 may also be used as a gas phase electrolyticconductivity detector (GPELCD) simply by inserting an anode and acathode. As shown in FIG. 7, normally thermocouple 406 may be used asthe cathode. The cathode is introduced through a quartz tube 420 and acatalyst material 422, such as mullite or alumina. The anode 424, whichis a platinum wire, is wound on the catalyst 422. The gas inlet isplaced at the left end of FIG. 7 at position 426, and the gas outlet isat position 428. The normal gas composition that comes out of a FID on agas chromatograph may be introduced into the detector of FIG. 7 atposition 426, with or without changing the gas phase composition.

FIG. 8 shows a combination of a GPELCD and a FID, taking the regularoutput gas from the FID directly into the GPELCD without modification.In the combination 600 of FIG. 8, the FID 602 is leak sealed at 604 tothe input of fused quartz or alumina tube 612 in the GPELCD furnace 606.The furnace 606 has a special heating coil 608 that gives very littleelectrical field effect inside the tube 612. Thermocouple and metalelectrodes are used as the temperature controller and gas phase currentelectrodes in detector circuits 614 connected to 6-wire output connector616. The wires of connector 616 are insulated in fused quartz. Acatalyst of alumina or mullite is provided at the center of the furnace606, which is controlled to 1° C. in the temperature range of 700° to1100° C. Mounting supports 610 are provided on either side of the GPELCDfurnace 606. Alternatively, the FID 602 could be replaced by aphotoionization detector or by the direct output of a gas chromatograph.

FIGS. 9A and 9B show experimental data for the combination of the FID602 and the GPELCD 606 in FIG. 8. Curves 700 and 702 are for standardscontaining compounds with and without chlorine present. The ten peaks1-10 in the curves 700 and 702, with some of the peaks present in bothcurves, are the solvent (methanol), methylene chloride, acetone,chloroform, methylchloroform, trichloroethylene, benzene, hexane,tetrachloroethylene and toluene, respectively. The chlorinated compoundsgave very small peaks in the FID 602. On the other hand, the GPELCD 606gave no peaks at all for the non-chlorine containing compounds. Themounts of the compounds in these curves are in the nanogram range,except for the methanol solvent, which gave no peak at all in theGPELCD.

The results of three standard runs and one air sample are shown in FIGS.10A-10D, using a combination of FID 602 and GPELCD 606 in FIG. 8. Theseven peaks in curves 800, 802 and 804 (FIGS. 10A-10C) are, from left toright, methylene chloride at 806A-806C, chloroform at 808A-808C, freonat 810A-810C, carbon tetrachloride at 812A-812C, trichloroethylene at814A-814C, tetrachloroethylene at 816A-816C and chlorobenzene at818A-818C. The GPELCD 606 gave no peaks at all for methanol, acetone,benzene, hexane and toluene, the non-chlorine containing compounds thatwere present in curves 800, 802 and 804, which were 0.5, 1 and 2microliters, respectively, of the standard solution in methanol. Theamounts of the compounds in-these curves are in the nanogram range,except for the solvent that entered the detector in one to three minutesand gave no peak at all in the GPELCD 606. The curves 800, 802 and 804show that the response of the GPELCD 606 is linear for these compounds.Curve 820 represents 174 liters of outdoor air in Menlo Park, Calif. onthe night of Oct. 7-8, 1992, collected on a special thermal-desorbablecharcoal tube. It contained additional chloro-compounds, such as methylchloroform at peak 822, freon 11 at peak 824, and several others not yetidentified. The levels are in the 10 to 500 pans per trillion, on a gasvolume basis.

The gravity electrolyte flow controlled electrochemical potentiometric,coulometric, and conductimetric detectors use flow rates approximately,but not limited to, 2 to 0.1 ml per minute with gas flow rates in therange of 10 to 1000 ml per minute. All three types of detectors havesensitivities of picograms per second for elements such as sulfur andhalogens.

It should further be apparent to those skilled in the an that variouschanges in form and details of the invention as shown and described maybe made. It is intended that such changes be included within the spiritand scope of the claims appended hereto.

What is claimed is:
 1. An electrochemical detection cell, whichcomprises a gravimetric electrolyte input flow capillary tubeintersecting a second capillary tube at a given point, a gas streaminput flow capillary tube for a gas containing a substance to bedetected, said gas stream input flow capillary tube entering said secondcapillary tube proximate to the given point, said second capillary tubeforming a mixing zone at the given point, at least one sensor electrodepositioned in said second capillary tube spaced from and beyond thegiven point, and an electrolyte reservoir connected to said gravimetricelectrolyte input flow capillary tube and positioned above saidgravimetric electrolyte input flow capillary tube, said second capillarytube having an exit for the electrolyte and the gas stream beyond saidat least one sensor electrode, the exit of said second capillary tubebeing connected to said electrolyte reservoir.
 2. The electrochemicaldetection cell of claim 1 additionally comprising a reference electrodein said gravimetric electrolyte input flow capillary tube.
 3. Theelectrochemical detection cell of claim 1 in which said at least onesensor electrode comprises a pair of sensor electrodes.
 4. Theelectrochemical detection cell of claim 1 in which said gas stream inputflow capillary tube is formed from a non-wettable plastic.
 5. Theelectrochemical detection cell of claim 2 in which said referenceelectrode is a silver/silver halide electrode.
 6. The electrochemicaldetection cell of claim 2 in which said at least one sensor electrodecomprises a pair of sensor electrodes.
 7. The electrochemical detectioncell of claim 6 additionally comprising a pair of generator electrodes,an additional sensor electrode and an additional reference electrode insaid electrolyte reservoir.
 8. The electrochemical detection cell ofclaim 1 additionally comprising a reference capillary tube connectedbetween said gravimetric electrolyte input flow capillary tube and saidsecond capillary tube above said at least one sensor electrode, and areference electrode in said reference capillary tube.
 9. Theelectrochemical detection cell of claim 8 additionally comprising abypass capillary tube connected between said gravimetric electrolyteinput flow capillary tube and said second capillary tube above said atleast one sensor electrode, a first generator electrode in saidgravimetric electrolyte input flow capillary tube, and a secondgenerator electrode in said bypass capillary tube.
 10. Theelectrochemical detection cell of claim 9 in which said at least onesensor electrode comprises a pair of sensor electrodes.
 11. Incombination, the electrochemical detection cell of claim 1 and apyrolysis furnace connected to said gas stream input flow capillary tubeto provide the gas.
 12. The combination of claim 11 in which saidpyrolysis furnace comprises first and second concentrically positionedelectrically insulating tubes, and a resistance heater wound around saidfirst electrically insulating tube between said first electricallyinsulating tube and said second electrically insulating tube.
 13. Thecombination of claim 12 in which said heating coil is wound from a firstend of said first electrically insulating tube to a second end of saidfirst electrically insulating tube and back again to the first end, saidpyrolysis furnace additionally comprising fused electrically insulatingpowder between said first and second electrically insulating tubes toprevent shorting of said heating coil.
 14. An electrochemical detectionmethod which comprises supplying an electrolyte solution through agravimetric electrolyte input flow capillary tube from an electrolytereservoir positioned above said capillary tube to a mixing zone at aflow rate controlled by the capillary tube, supplying a gas containing adetectable substance through a gas input capillary tube to the mixingzone, mixing the electrolyte solution and the gas in the mixing zone,passing the electrolyte solution and gas from the mixing zone to adetection zone, detecting the detectable substance in the electrolytesolution within the detection zone, and flowing the electrolyte solutionand gas from the detection zone into the reservoir.
 15. Theelectrochemical detection method of claim 14 in which the detectablesubstance is detected potentiometrically.
 16. The electrochemicaldetection method of claim 14 in which the detectable substance detectedcoulometrically.
 17. The electrochemical detection method of claim 14 inwhich the detectable substance is detected conductimetrically.
 18. Theelectrochemical detection method of claim 14 in which the gas containinga detectable substance is supplied from a pyrolysis furnace.
 19. Anelectrochemical detection method which comprises supplying anelectrolyte solution through a gravimetric electrolyte input flowcapillary tube from an electrolyte reservoir to a mixing zone at a flowrate controlled by the capillary tube, supplying a gas containing adetectable substance through a gas input capillary tube to the mixingzone, mixing the electrolyte solution and the gas in the mixing zone,passing the electrolyte solution and gas from the mixing zone to adetection zone, detecting the detectable substance in the electrolytesolution within the detection zone, and flowing the electrolyte solutionand the gas from the detection zone into the reservoir.
 20. Theelectrochemical detection method of claim 19 in which the electrolytesolution and gas are separated in a gas-liquid separator containingsensor electrodes after the mixing zone and before the gas and theelectrolyte solution are flowed into the reservoir.
 21. A pyrolysisfurnace for use in an electrochemical detection cell which comprisesfirst and second concentrically positioned electrically insulatingtubes, and a resistance heating coil wound in a first direction from afirst end of said first electrically insulating tube to a second end ofsaid first electrically insulating tube and back again in the firstdirection to the first end, said heating coil mounted between said firstelectrically insulating tube and said second electrically insulatingtube.
 22. The pyrolysis furnace of claim 21 in which said pyrolysisfurnace additionally comprises fused electrically insulating powderbetween said first and second electrically insulating tubes to preventshorting of said heating coil.
 23. The pyrolysis furnace of claim 22additionally comprising a thermocouple in said first electricallyinsulating tube.
 24. The pyrolysis furnace of claim 23 in which saidpyrolysis furnace comprises a gas phase electrolytic conductivitydetector, said thermocouple comprises a cathode of said gas phaseelectrolytic conductivity detector, and said pyrolysis furnaceadditionally comprises an anode of said gas phase electrolyticconductivity detector.
 25. The pyrolysis furnace of claim 23additionally comprising a layer of thermally insulating material on saidsecond electrically insulating tube.
 26. The pyrolysis furnace of claim24 in combination with a source of a chlorine containing sampleconnected to said second electrically insulating tube and a source of agas for reacting with the chlorine containing sample to produceionizable chlorine.
 27. The pyrolysis furnace of claim 24 in combinationwith a source of a sample containing an ionizable element other thancarbon, nitrogen and hydrogen connected to said second electricallyinsulating tube.
 28. The combination of claim 27 in which said source ofa sample containing an ionizable element comprises a flame ionizationdetector, a photoionization detector or a direct output of a gaschromatograph.
 29. The combination of claim 28 in which the flameionization detector, photoionization detector or direct output of a gaschromatograph is a source of a sample containing chlorine, bromine,sulfur, nitrogen or phosphorus.
 30. The pyrolysis furnace of claim 28 inwhich said source of a sample containing an ionizable element comprisesa gas chromatograph.
 31. The combination of claim 26 in which saidsource of the gas is an oxygen, hydrogen or mixture of oxygen andhydrogen source.
 32. The combination of claim 26 in which said source ofa chlorine containing sample comprises a flame ionization detector, aphotoionization detector or a direct output of a gas chromatograph.